1. Field of the Invention
The present invention relates to a light-receiving device including a built-in circuit for processing a photoelectrically converted signal (hereinafter, referred to as a "circuit-integrated light-receiving device"). More particularly, the present invention relates to a circuit-integrated light-receiving device having a capability of improving the response speed of a photodiode which generates the photoelectrically converted signal based on incident light.
2. Description of the Related Art
In recent years, an optical disk apparatus is required to process a large amount of data such as video data at a high speed. For example, an optical disk apparatus for use with a DVD (a DVD-ROM apparatus) has been rapidly improved in terms of the data read speed (e.g., from a normal-speed drive to a double-speed drive). In the future, an optical disk apparatus with an even faster data read speed (e.g., a 12.times.-speeddrive) will be demanded. A DVD-ROM apparatus typically uses an optical pick-up chip for reading out signals. The optical pick-up chip includes, on the same chip, a light-receiving device and a signal processing circuit for processing a photoelectrically converted signal from the light-receiving device. In order to further increase the operating speed of a DVD-ROM apparatus, there is a demand to increase the operating speed of the light-receiving device which is included in such an optical pick-up (more generically a "circuit-integrated light-receiving device").
Conventionally, a light-receiving device included in an optical pick-up employs a PN junction between an N-type epitaxial (semiconductor crystal growth) layer and a P-type substrate, or a PN junction between an N-type epitaxial layer and a P-type diffusion layer. However, when the former type of PN junction between an N-type epitaxial layer and a P-type substrate is used, a photo carrier generated in the substrate moves by diffusion, thereby reducing the response speed. On the other hand, when the latter PN junction between an N-type epitaxial layer and a P-type diffusion layer is used, the junction capacitance increases according to the impurity concentration in the N-type epitaxial layer, thereby also reducing the response speed. Moreover, when the latter PN junction is used in a DVD apparatus, a major portion of the laser light having a wavelength of 650 nm which is used by the DVD apparatus as reproduction light goes into the substrate, thereby reducing the operational sensitivity.
As described above, the conventional circuit-integrated light-receiving device is likely to have poor operational characteristics as compared with a pin photodiode which does not include a built-in circuit.
In order to solve these problems, a number of structures have been proposed in the art.
FIG. 26 illustrates a structure which is disclosed in Japanese Laid-Open Publication No. 61-154063. In this structure, a P-type epitaxial layer 142 is provided on the surface of a P.sup.+ -substrate 141. The P-type epitaxial layer 142 includes a P-type high-impurity concentration layer (auto-doped layer) 142a and a P-type low-impurity concentration layer 142b. The P-type high-impurity concentration layer 142a is provided by an upward diffusion (auto-doping) of an impurity from the substrate 141 which occurs during the growth of the P-type epitaxial layer 142.
An N-type epitaxial layer 143 is provided on the P-type epitaxial layer 142. A P.sup.+ -separation diffusion layer 144 having a high impurity concentration extends from the upper surface of the N-type epitaxial layer 143 to the underlying P-type epitaxial layer 142. The separation diffusion layer 144 divides the N-type epitaxial layer 143 into a number of regions and separates the regions from one another.
Some of the separated regions of the N-type epitaxial layer 143 each form a light-receiving device section 180. In particular, the light-receiving device section 180 includes a PN junction formed between one of the separated regions of the N-type epitaxial layer 143 and the underlying P-type epitaxial layer 142. Each of the other ones of the separated regions of the N-type epitaxial layer 143 which is adjacent to the light-receiving device section 180 forms a signal processing circuit section (NPN transistor) 190. In the illustrated example, the signal processing circuit section (NPN transistor) 190 includes a buried region 165 for reducing the collector resistance, a base region 147 and an emitter region 148. The light-receiving device section 180 and the signal processing circuit section 190 are electrically separated from each other by the separation diffusion layer 144.
An oxide layer 149 is provided on the upper surface of each of these structures. An electrical line layer 150a is connected to the contact region 145 of the light-receiving device section (photodiode) 180 via a contact hole provided in the oxide layer 149. An electrical line layer 150b and an electrical line layer 150c are connected to the signal processing circuit section (NPN transistor) 190 similarly via a contact hole. The electrical line layer 150b is also connected to the separation diffusion layer 144.
As described above, the structure illustrated in FIG. 26 includes the substrate 141 having a high impurity concentration and the P-type epitaxial layer 142 which has a lower impurity concentration. Thus, the depletion layer on the side of the P-type semiconductor which forms the photodiode (a region denoted by a one-dot chain line) is substantially extended into the P-type epitaxial layer 142, thereby reducing the junction capacitance of the photodiode 180. Due to the extension of the depletion layer, a photo carrier generated in a deep location can sufficiently contribute to the photoelectric current.
Moreover, a P-type high-impurity concentration layer (auto-doped layer) 142a included in this structure has a concentration gradient which gradually decreases in the upward direction from the substrate 141. A potential gradient is produced by the concentration gradient, which in turn generates an internal electric field, whereby it is possible to move at a high speed a photo carrier that is generated in a deep location (lower portion) of the P-type epitaxial layer 142.
Next, FIG. 27 illustrates a structure which is disclosed in Japanese Laid-Open Publication No. 4-271172. In the structure, a non-doped first epitaxial layer 224 is provided on a P-type substrate 223, and a P-type well region 226 is formed in a portion of the non-doped first epitaxial layer 224 corresponding to the location where a signal processing circuit section (NPN transistor) 290 is provided. An N-type second epitaxial layer 225 is provided on the first epitaxial layer 224. An N.sup.+ -diffusion region 230 is provided in the light-receiving device section (photodiode) 280 in the vicinity of the surface of the N-type second epitaxial layer 225. Regions 235, 236 and 237 of the NPN transistor are provided in the signal processing circuit section 290 in the vicinity of the surface of the N-type second epitaxial layer 225. An N.sup.+ -diffusion region 234 is provided below the regions 235, 236 and 237. The signal processing circuit section 290 and the photodiode section 280 are electrically separated from each other by a separation diffusion region 227 including two regions 228 and 229.
An oxide layer 231 is provided on the surface of each of the structures. Electrical line layers 232 and 233 are connected to the light-receiving device section (photodiode) 280 via a contact hole provided in the oxide layer 231. An electrical line layer 238 is connected to the signal processing circuit section (NPN transistor) 290 similarly via a contact hole.
The structure illustrated in FIG. 27 employs the substrate 223 having a specific resistance of about 40 .OMEGA.cm to about 60 .OMEGA. so as to control the auto-doping process from the substrate 223 to the overlying first epitaxial layer 224. Moreover, a non-doped semiconductor crystal layer is used as the overlying first epitaxial layer 224, whereby the depletion layer in the photodiode section 280 can extend by a substantial distance toward the substrate. Furthermore, the P-type well region 226 is provided, so that the NPN transistor is surrounded by the P-type regions, i.e., the separation diffusion region 227 (228 and 229) and the P-type well region 226, whereby it is possible to reduce the parasitic effect.
Next, FIG. 28 illustrates a structure which is disclosed in Japanese Laid-Open Publication No. 1-205564. The structure includes a P-type epitaxial layer 320 formed on the surface of a P.sup.+ -substrate 310. The P-type epitaxial layer 320 includes a P-type auto-doped layer 321 and a P-type low-impurity concentration layer 322. The P-type auto-doped layer 321 is provided by an upward diffusion (auto-doping) of an impurity from the substrate 310 which occurs during the growth of the P-type epitaxial layer 320.
An N-type epitaxial layer 330 is provided on the P-type epitaxial layer 320. A P.sup.+ -separation diffusion region 340 having a high impurity concentration extends from the upper surface of the N-type epitaxial layer 330 into the auto-doped layer 321 of the P-type epitaxial layer 320. The separation diffusion region 340 divides the N-type epitaxial layer 330 into a number of regions and separates the regions from one another.
Some of the separated regions of the N-type epitaxial layer 330 each form a light-receiving device section 380. In particular, the light-receiving device section 380 includes a PN junction formed between one of the separated regions of the N-type epitaxial layer 330 and the underlying P-type epitaxial layer 320. An N.sup.+ -type diffusion layer 334 which functions as a light receiving surface electrode extends over a relatively large area in the light-receiving device section 380 in the vicinity of the N-type epitaxial layer 330. Each of the other ones of the separated regions of the N-type epitaxial layer 330 which is adjacent to the light-receiving device section 380 forms a signal processing circuit section (NPN transistor) 390. In the illustrated example, the signal processing circuit section (NPN transistor) 390 includes a buried region 323 for reducing the collector resistance, a P-type diffusion layer 331 and an N.sup.+ -type diffusion layer 333. The light-receiving device section 380 and the signal processing circuit section 390 are electrically separated from each other by the separation diffusion region 340.
An insulation film 335 is provided on the upper surface of each of these structures. Electrode and line elements 336 and 337 are electrically connected to predetermined locations of the light-receiving device section 380 and the signal processing circuit section 390 via contact holes provided in the insulation film 335.
In the structure illustrated in FIG. 28, the light-receiving device section 380 and the adjacent signal processing circuit section 390 are electrically separated from each other by the deep separation diffusion region 340. As a result, the depletion layer formed in the light-receiving device section 380 can extend by a substantial distance toward the substrate (i.e., into the auto-doped layer 321 of the P-type epitaxial layer 320) without extending into other adjacent photodiode and signal processing circuit sections.
Typically, the response characteristic of a photodiode is dependent on the Junction capacitance provided by the PN junction and the series resistance which is determined by the resistance component of each portion of the photodiode.
Among others, the junction capacitance is basically determined by the impurity concentration of the substrate. Therefore, the junction capacitance can generally be improved by using a high-specific-resistance substrate having a low impurity concentration. In the conventional structures illustrated in FIGS. 26 to 28, the junction capacitance is improved either by suppressing the impurity concentration of the P-type epitaxial layer provided on the substrate or by increasing the resistance thereof by making the layer non-doped.
In the structures illustrated in FIGS. 26 and 27, the junction capacitance is improved as described above, but the series resistance is not sufficiently improved. This will be further described below.
Generally, it is believed that the series resistance of a photodiode includes the following components R1-R7:
R1: The resistance of the separation diffusion region PA1 R2: The resistance of the buried diffusion layer underlying the separation diffusion region PA1 R3: The resistance of the high-specific-resistance epitaxial layer underlying the separation diffusion region PA1 R4: The resistance of the auto-doped layer underlying the separation diffusion region PA1 R5: The substrate resistance PA1 R6: The resistance of the auto-doped layer underlying the photodiode section PA1 R7: The resistance of the high-specific-resistance epitaxial layer underlying the photodiode section PA1 (for R1-R7, see FIGS. 6A and 6B or FIGS. 17A and 17B).
The series resistance of the photodiode section of each of the conventional structures will now be discussed. In each of the conventional structures, the separation diffusion region has a high impurity concentration, whereby the resistance R1 is low. Moreover, the substrate has a high impurity concentration, whereby the substrate resistance R5 is low. The resistances R4 and R6 of the auto-doped layer provided by a diffusion of an impurity from the substrate do not significantly affect the series resistance. Moreover, judging from the structure of the buried diffusion layer, the resistance R2 of the buried diffusion layer either does not exist (FIGS. 26 and 28) or does not substantially contribute to the series resistance of the photodiode (FIG. 27).
However, in the structure illustrated in FIG.ure 26, the high-specific-resistance epitaxial layer 142b underlying the separation diffusion layer 144 has a low impurity concentration, whereby the resistance R3 thereof is high. Moreover, a portion of the high-specific-resistance epitaxial layer 142b underlying the separation diffusion layer 144 may be depleted by an influence of a bias voltage applied across the photodiode due to the low impurity concentration, thereby further increasing the resistance R3. This is true also in the structure illustrated in FIG. 27, where the resistance R3 of the non-doped first epitaxial layer 224 underlying the separation diffusion region 227 is high.
For the foregoing reasons, each of the conventional structures illustrated in FIGS. 26 and 27 has a reduced photodiode junction capacitance, but has a high series resistance due to the high resistance component of the low-concentration or non-doped P-type epitaxial layer underlying the N-type epitaxial layer, thereby lowering the response speed of the photodiode.
On the contrary, the structure illustrated in FIG. 28 employs the high-impurity concentration substrate 310, thereby reducing the substrate resistance R5, while employing the deep separation diffusion region 340 which reaches the auto-doped layer 321 having a high impurity concentration, thereby eliminating the resistance component R3. Moreover, the resistance component R7 underlying the photodiode section can similarly be eliminated by extending the depletion layer to the auto-doped layer 321. As a result, the structure overcomes the problem of a high series resistance, thereby improving the response speed.
However, when the separation diffusion region 340 is extended to such a depth, as in the structure illustrated in FIG. 28, the diffusion step diffuses the impurity in the lateral direction as well as the depth direction. Therefore, the width of the separation diffusion region 340 increases as well as the depth thereof. Such an increase in the lateral size of the separation diffusion region 340 will naturally increase the size of the entire device. This is undesirable in view of the increasing demand in the art to reduce the device size.
Moreover, when a separation diffusion region extends deeply in the structure, as schematically illustrated in FIG. 29B, the distance by which a photo carrier generated under the separation diffusion region moves naturally increases as compared to the case of a shallow separation diffusion region as illustrated in FIG. 29A, thereby lowering the response speed of the photodiode. The problem due to the formation of the deep separation diffusion region is particularly pronounced when the structure is used in a split photodiode, as discussed in, for example, Japanese Laid-Open Publication No. 8-32100.
Moreover, in the conventional structure illustrated in FIG. 28, the separation diffusion region 340, which contributes to the reduction in the series resistance of the photodiode, is provided only in the device separation portion. Therefore, it is necessary to increase the impurity concentration of the separation diffusion region 340 in order to reduce the resistance value. In particular, in order to obtain an impurity concentration of about 1.times.10.sup.16 atoms/cm.sup.3 in the vicinity of the boundary between the separation diffusion region 340 and the auto-doped layer 321, it is necessary to set the impurity concentration on the surface of the P-type buried diffusion layer of the separation diffusion region 340 in a range of about 1.times.10.sup.18 atoms/cm.sup.3 to about 1.times.10.sup.19 atoms/cm.sup.3.
When forming the N-type epitaxial layer 330, the impurity on the surface of the P-type buried diffusion layer of the separation diffusion region 340 is auto-doped, thereby forming an auto-doped layer. The impurity concentration of such an auto-doped layer is typically about 10.sup.-3 of that of the auto-dope source. In the example illustrated in FIG. 28, the impurity concentration on the surface of the P-type buried diffusion layer of the separation diffusion region 340 is about 1.times.10.sup.18 atoms/cm.sup.3 to about 1.times.10.sup.19 atoms/cm.sup.3, whereby an auto-doped layer formed on the surface of the P-type epitaxial layer 320 has an impurity concentration of about 1.times.10.sup.16 atoms/cm.sup.3. In the P-type epitaxial layer 320, which forms the PN junction of the photodiode, the impurity concentration in the vicinity of the PN junction is preferably about 1.times.10.sup.13 atoms/cm.sup.3 to about 1.times.10.sup.14 atoms/cm.sup.3 in order to obtain a reduced junction capacitance. Therefore, when an auto-doped layer having a high impurity concentration as described above exists in the vicinity of the PN junction, the extension of the depletion layer is restricted, thereby increasing the junction capacitance and thus lowering the response speed of the photodiode.
As schematically illustrated in FIGS. 30A and 30B, the auto-doped layer provided in the vicinity of the PN junction also has a substantial influence on the movement of a carrier (electron) generated in the P-type substrate.
In particular, if no auto-doped layer exists on the surface of the P-type substrate (herein, it is assumed that the P-type substrate also includes the P-type epitaxial layer formed on the substrate), i.e., in the vicinity of the PN junction, a carrier (electron) generated in the P-type substrate can move into the N-type epitaxial layer without having to overcome a barrier, as illustrated in FIG. 30A. However, if an auto-doped layer exists on the surface of the P-type substrate (in the vicinity of the PN junction), the auto-doped layer acts as a potential barrier for an electron, thereby restricting the movement of the electron from inside the P-type substrate to the N-type epitaxial layer, as illustrated in FIG. 30B, thereby lowering the response speed of the photodiode. Therefore, unless the impurity concentration on the surface of the P-type buried diffusion layer of the separation diffusion region 340 is set to a level such that an auto-doped layer will not be formed in the vicinity of the PN junction by autodoping, the response speed of the photodiode cannot be improved sufficiently.
As described above, it has not been possible in the prior art to obtain a structure capable of achieving a sufficiently high photodiode response speed, while reducing the junction capacitance of the photodiode and the series resistance.